There are believed to be rotation rate sensors that have caverns in which different pressures are enclosed.
FIG. 1 shows a such a micromechanical sensor device 100. A very low pressure is enclosed in cavern 3, typically 1 mbar. The background for this measure is that some of movable MEMS structures 5, 6 are resonantly driven in these sensors. At a low pressure, it is thus easy to excite oscillations using relatively low voltages due to the low damping.
In the case of acceleration sensors, in contrast, it is not desirable that the sensor begins to oscillate, which would be possible if an externally present oscillation or external pulse-shaped acceleration were present. These sensors are therefore operated at higher internal pressures, typically at approximately 500 mbar.
If very small and cost-effective combinations of rotation rate and acceleration sensors are to be created, this may be carried out by providing both an MEMS structure 5 of a rotation rate sensor and an MEMS structure 5 of an acceleration sensor on one chip of sensor device 100. The two sensors are created simultaneously on one substrate 1. The sensors are encapsulated at the substrate level with the aid of a cap wafer 2, which provides two caverns 3, 4 per chip.
The different pressures which are required in caverns 3, 4 of the rotation rate sensor and of the acceleration sensor may be achieved by using a getter layer 6, for example. A getter layer 6 is locally situated in cavern 4 of the rotation rate sensor. A high pressure is then enclosed in the two caverns 3, 4. Subsequently, getter layer 6 is activated with the aid of a temperature step, whereby getter layer 6 then “pumps,” or getters, the cavern volume over the rotation rate sensor to a lower pressure.
However, this method is comparatively complex and expensive. A porous layer having a very large surface must be created as getter layer 6, which initially must be sealed. Getter layer 6 must not be activated until caverns 3, 4 have been sealed; this means that the seal is broken open with the aid of a temperature step.
Moreover, manufacturing methods for rotation rate sensors and acceleration sensors including an integrated evaluation circuit (CMOS circuit), which is shown schematically in FIG. 2, for example, are known. For example, a structured oxide layer 11 is provided on a first substrate 10 in the form of a CMOS wafer, onto which a monocrystalline functional layer is bonded. An electrical connection 7 is established between the functional layer and the CMOS circuit with the aid of a trench process and filling with a conductive material. The functional layer is structured via a further trench step, and free-standing MEMS structures 5 are created, whose movement may be capacitively excited or detected, for example.
The functional layer is subsequently hermetically sealed with a cap wafer 2. Depending on the application, a suitable pressure is enclosed within the closed volume. Contact surfaces with first substrate 10 are exposed to relay the ASIC signal via bonding wires and a suitable housing. A corresponding method is known from US 20100109102 A1, for example.
If very small sensors are to be created, a housing may also be omitted, and the ASIC MEMS chip may be directly soldered onto a printed circuit board, whereby a so-called bare die system is implemented. To establish the direct solderability, however, the ASIC signal must be conducted through the ASIC wafer to the ASIC wafer rear side via an electrical via 12. There, a rewiring plane 13 is usually provided to then transmit the ASIC signal via solder bumps 14, which are also provided on the rear side, to the printed circuit board (not shown). A corresponding sensor is known from US 20120049299 A1, for example.
It is also possible, however, to manufacture the vias 12 shown in FIG. 2 so that they are only very thin for technological reasons, as a result of which the entire first substrate 10 has only a very thin configuration. The free-standing MEMS structures are anchored on first substrate 10 in the form of the ASIC wafer. Since the ASIC wafer is very thin, external mechanical stresses which are caused by different coefficients of expansion of the printed circuit board and of the ASIC wafer may easily bend MEMS structures 5 and thus lead to strong false signals in the sensor.